This week, Congress debates an immigration bill, a plan by the bipartisan Gang of Eight, as they're called. The Senate Judiciary Committee begins reviewing the legislation on Thursday.

GREENE: Part of the debate is between Republicans and Democrats. But maybe the most vital debate is between conservatives.

INSKEEP: Many Republicans have said their party must approve immigration reform. Years of opposition cost them massively at election time, especially in 2012. Yet many Republicans also remain deeply skeptical of granting legal status or a path to eventual citizenship to estimated 11 million people in the U.S. illegally.

GREENE: Yesterday, the influential think tank, the Heritage Foundation, warned of the cost of that move. We begin our coverage with NPR's David Welna.

DAVID WELNA, BYLINE: The Heritage Foundation is the first group out of the starting gates to deliver a cost-benefit analysis of the Gang of Eight's immigration bill. That report had been awaited with some anticipation since the multi-trillion dollar price tag the foundation put on the last attempt to overhaul immigration some six years ago is credited with sinking that bill. At yesterday's unveiling of the report, lead author Robert Rector took pains to clarify what it is not.

ROBERT RECTOR: It is not an analysis of the entire immigration reform bill, which is something I hope to do in the future. But this report focuses primarily on amnesty.

WELNA: Or what others more friendly to the bill call a path to citizenship for most of those here illegally. The report does find some benefits to having millions more people registered as taxpayers. But those benefits, according to Rector, are outweighed by the costs over the next 50 years.

RECTOR: The amnesty recipients will receive $9.4 trillion in government benefits and services. They will pay $3.1 trillion in taxes for a net deficit of $6.3 trillion. That $6.3 trillion has to be paid by someone else, by the U.S. taxpayer.

WELNA: That calculation is premised on a broad assumption; namely, that because more than half of the unlawful immigrants have not finished high school, their households would eventually get $35,000 more in government benefits each year than what they pay in taxes. Rector says that's the average fiscal deficit for American households headed by someone lacking a high school diploma.

RECTOR: And what the amnesty proponents are saying is that they can take someone from Mexico or Guatemala with a 10th grade education, plunk them into that system, and somehow miraculously that individual is going to pay more in taxes than they take out in benefits. That's not only untrue, it's just profoundly implausible.

WELNA: Critics, though, are not convinced. The libertarian Cato Institute dismissed the Heritage report as fatally flawed. Others accused the foundation of spreading the politics of fear.

DOUGLAS HOLTZ-EAKIN: They had to go out 50 years to get a big number. It just looks pretty transparent.

WELNA: That's Douglas Holtz-Eakin. During the last Bush administration he headed the Congressional Budget Office, which looks at 10-year periods when calculating the cost of a bill. He says it's not big news that education influences how much or how little households depend on government aid.

HOLTZ-EAKIN: If you want to say, do people with less than high school educations pay less in taxes, get more in benefits than does the rest of the population, you and I know that answer without doing anything. So this is really about the number and the size of that impact for a particular population. And I think he's overstated it.

WELNA: Holtz-Eakin also faults the study for failing to take into account the upward mobility of striving immigrants, even those with little formal education. He and others plan to release their own cost-benefit analyses of the immigration bill soon.